Objective  Coastal mangrove wetland is an important part of coastal ecosystem, which can provide important ecosystem services such as biodiversity conservation, wind and wave prevention, water purification, and coastal blue carbon. However, due to land reclamation, deforestation, pond farming, port and industrial zone construction, and global climate change, mangrove wetland has become a coastal wetland featuring drastically changed ecological environment and seriously damaged ecosystem. In recent years, although many researches have been conducted on the ecological restoration of mangrove wetlands, they mostly focus on the restoration of the morphological structure of mangrove wetlands, while seldom involving the overall function or the coordination and coupling between various ecological functions of mangrove wetlands. How to propose a reproducible multi-functional coupling design technology for mangrove wetlands while restoring the overall structure thereof is an important issue that needs to be urgently solved in the current research on innovative ecological restoration of coastal mangrove wetlands.

Methods  This research adopts the case study approach. Taking mangrove wetland restoration in Sanjiang Bay, Haikou City as an example, this research, based on the analysis of background conditions and degradation status, as well as the demand for multi-functional ecological restoration objectives such as dewatering, proposes a strategy framework for coastal mangrove ecology restoration with multi-functional coupling design as its core, namely the "AMSSDS" strategy framework composed of the "Adaptive strategy", "Multi-functional coupling strategy", "Self-design of nature strategy", "Spatial heterogeneity design strategy", "Dynamic design strategy", and "Synergistic symbiosis strategy".

Results  The research summarizes three restoration models for coastal mangrove wetlands according to tidal changes, which can be described in details as follows. 1) Mangrove community restoration model: Create mudflats suitable for afforestation by returning pond to wetland, and further plant mangrove plants to restore and rebuild healthy and stable mangrove communities. 2) Mangrove & dike-pond restoration model: Create a mangrove & dike-pond composite ecosystem through the rational use of existing artificial fish ponds. 3) Mangrove tidal creek network construction and restoration model: Through dredging existing ditches and rivers and connecting reservoirs, repair the tidal creek system by stages to restore the mangrove tidal creek network system. The above three restoration models emphasize the combination of plant, water, soil and other major components with structural restoration and the integration of environmental and biological elements, and provide a spatial carrier for multi-functional coupling design with biodiversity as the core, linking biodiversity conservation with coastal protection, coastal blue carbon, wetland economy and other ecological functions to realize multi-functional collaborative optimization of coastal mangrove wetlands.

Conclusion  Based on existing fish ponds, tidal creeks and vegetation in the mangrove wetland in Sanjiang Bay, Haikou City, and following the principle of giving priority to the dominant functions while attaching equal importance to multiple functions of the ecosystem services of coastal mangrove wetlands, the research explores various environmental and biological elements and the overall collaboration thereof in mangrove wetland restoration, and discusses the multi-functional coupling design path with biodiversity conservation as the core. There are three main integrated design paths, which can be described in detail as follows. 1) Multi-functional habitat coupling design involving benthos, fish, mangrove plants and birds: According to the ecological types, living habits and needs for mangrove habitats of birds, and in combination with the topography, tidal hydrology, tidal creek landform, benthic animals and fish habitats in the mangrove wetland in Sanjiang Bay, construct a mangrove wetland life paradise with high habitat and landscape heterogeneity and stable ecosystem through restoring the tidal flat topography, building a tidal creek system, constructing bird islands, and rationally allocating plants based on the mangrove & dike-pond restoration model. 2) The coupling design of biodiversity conservation, coastal blue carbon function improvement and coastal protection function enhancement: In the mangrove ecology restoration area in Sanjiang Bay, the design forms a mangrove community with complex structure and ensures high biomass through the mixing of diversified true mangrove and semi-mangrove plant species. The mangrove tidal flat benthic system and mangrove communities with different tide levels and elevations are constructed to form mangrove multi-habitat system and three-dimensional carbon sequestration system from below to above water, so as to realize the synergy of biodiversity conservation and coastal blue carbon function enhancement. Meanwhile, a stable mangrove network life support system is formed, which can strengthen such functions as wind and wave prevention, and wave reduction and flow slowing in the coastal zone. 3) Coupling design of biodiversity conservation and sustainable wetland economy development: The mangrove system in the mangrove restoration area in Sanjiang Bay is utilized to develop the mangrove characteristic aquaculture industry according to local conditions. For example, the sipunculid is put into the mangrove & dike-pond system, a sustainable three-dimensional coupling system of mangrove plantation and aquaculture is established, and the high-quality ecological aquaculture of aquatic products is implemented. In addition, the mangrove plant medication and bee farming industries can be developed in collaboration with coastal ecological tourism to promote the sustainable growth of the coastal economy. Multi-functional coupling design is an important basis for comprehensive optimization and improvement of ecological service functions in the restoration of coastal mangrove wetland ecosystem, which can provide a scientific basis and practical reference for future restoration of coastal mangrove wetlands.